The imaging system of a digital camera, a video camera, or the like conventionally includes an image sensor and a mechanism that adjusts the amount of light incident on the image sensor. An example of the mechanism that adjusts the light amount is a stop device. The stop device drives diaphragm blades and adjusts the opening diameter in accordance with the luminance of an object, thereby adjusting the amount of light incident on the image sensor. When the luminance of the object is high, the opening diameter needs to be small. However, when the opening has a predetermined diameter or less, image quality may degrade due to a hunting phenomenon or diffraction of light. A recent image sensor has a high sensitivity, and the demand for further light amount suppression is growing.
For this purpose, a light attenuation filter having an almost even transmittance within the wavelength range of visible light (400 to 700 nm), for example, an ND (Neutral Density) filter is used. More specifically, when the ND filter is arranged near the diaphragm blades, the amount of light incident on the image sensor can be attenuated while maintaining the opening diameter. There are, for example, an ND filter made by mass-coloring glass or plastic with a pigment or dye and an ND filter formed by stacking a plurality of inorganic films on a substrate by vacuum deposition, sputtering, or the like. Considering the optical characteristic and the environmental characteristic, the latter ND filter is generally used in the imaging optical system.
There has been developed a technology of providing a plurality of ND filters in the imaging optical system and switching the ND filter to be inserted into the optical path in accordance with the luminance of an object (Japanese Patent Laid-Open No. 2001-222042). Also developed is a technology of providing regions of different light transmittances in one ND filter and switching the region to be inserted into the optical path in accordance with the luminance of an object (Japanese Patent Laid-Open No. 2004-295015).